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问题:
It looks like the lists returned by keys() and values() methods of a dictionary are always a 1-to-1 mapping (assuming the dictionary is not altered between calling the 2 methods).
For example:
>>> d = {'one':1, 'two': 2, 'three': 3} >>> k, v = d.keys(), d.values() >>> for i in range(len(k)): print d[k[i]] == v[i] True True True
If you do not alter the dictionary between calling keys() and calling values(), is it wrong to assume the above for-loop will always print True? I could not find any documentation confirming this.
回答1:
Found this:
If items(), keys(), values(), iteritems(), iterkeys(), and itervalues() are called with no intervening modifications to the dictionary, the lists will directly correspond.
On 2.x documentation and 3.x documentation.
回答2:
Yes, what you observed is indeed a guaranteed property -- keys(), values() and items() return lists in congruent order if the dict is not altered. iterkeys() &c also iterate in the same order as the corresponding lists.
回答3:
Yes it is guaranteed in python 2.x:
If keys, values and items views are iterated over with no intervening modifications to the dictionary, the order of items will directly correspond.
回答4:
According to http://docs.python.org/dev/py3k/library/stdtypes.html#dictionary-view-objects , the keys(), values() and items() methods of a dict will return corresponding iterators whose orders correspond. However, I am unable to find a reference to the official documentation for python 2.x for the same thing.
So as far as I can tell, the answer is yes, but only in python 3.0+
回答5:
For what it's worth, some heavy used production code I have written is based on this assumption and I never had a problem with it. I know that doesn't make it true though :-)
If you don't want to take the risk I would use iteritems() if you can.
for key, value in myDictionary.iteritems(): print key, value
回答6:
Yes. Starting with CPython 3.6, dictionaries return items in the order you inserted them.
The documentation hasn't been updated yet, ignore the part that says this is an implementation detail. This is already guaranteed in CPython 3.6 and will be required for all other Python implementations starting with Python 3.7.
回答7:
I wasn't satisfied with these answers since I wanted to ensure the exported values had the same ordering even when using different dicts.
Here you specify the key order upfront, the returned values will always have the same order even if the dict changes, or you use a different dict.
keys = dict1.keys() ordered_keys1 = [dict1[cur_key] for cur_key in keys] ordered_keys2 = [dict2[cur_key] for cur_key in keys]
